13.2213.22
Mass SpectrometryMass Spectrometry
Atom or molecule is hit by high-energy electronAtom or molecule is hit by high-energy electron
Principles of Electron-Impact Mass SpectrometryPrinciples of Electron-Impact Mass Spectrometry
ee––
Atom or molecule is hit by high-energy electronAtom or molecule is hit by high-energy electron
electron is deflected but transfers much of electron is deflected but transfers much of its energy to the moleculeits energy to the molecule
ee––
Principles of Electron-Impact Mass SpectrometryPrinciples of Electron-Impact Mass Spectrometry
Atom or molecule is hit by high-energy electronAtom or molecule is hit by high-energy electron
electron is deflected but transfers much of electron is deflected but transfers much of its energy to the moleculeits energy to the molecule
ee––
Principles of Electron-Impact Mass SpectrometryPrinciples of Electron-Impact Mass Spectrometry
This energy-rich species ejects an electron.This energy-rich species ejects an electron.
Principles of Electron-Impact Mass SpectrometryPrinciples of Electron-Impact Mass Spectrometry
This energy-rich species ejects an electron.This energy-rich species ejects an electron.
Principles of Electron-Impact Mass SpectrometryPrinciples of Electron-Impact Mass Spectrometry
forming a positively charged, odd-electron forming a positively charged, odd-electron species called the species called the molecular ionmolecular ion
ee––++••
Molecular ion passes between poles of a Molecular ion passes between poles of a magnet and is deflected by magnetic fieldmagnet and is deflected by magnetic field
amount of amount of deflection depends deflection depends on mass-to-charge on mass-to-charge ratioratio
highest m/z highest m/z deflected leastdeflected least
lowest m/z lowest m/z deflected mostdeflected most
Principles of Electron-Impact Mass SpectrometryPrinciples of Electron-Impact Mass Spectrometry
++••
Principles of Electron-Impact Mass SpectrometryPrinciples of Electron-Impact Mass Spectrometry
If the only ion that is present is the molecular If the only ion that is present is the molecular ion, mass spectrometry provides a way to ion, mass spectrometry provides a way to measure the molecular weight of a compound measure the molecular weight of a compound and is often used for this purpose.and is often used for this purpose.
However, the molecular ion often fragments to However, the molecular ion often fragments to a mixture of species of lower m/z.a mixture of species of lower m/z.
The molecular ion dissociates to a cationThe molecular ion dissociates to a cationand a radical.and a radical.
Principles of Electron-Impact Mass SpectrometryPrinciples of Electron-Impact Mass Spectrometry
++••
The molecular ion dissociates to a cationThe molecular ion dissociates to a cationand a radical.and a radical.
Principles of Electron-Impact Mass SpectrometryPrinciples of Electron-Impact Mass Spectrometry
++ ••
Usually several fragmentation pathways are Usually several fragmentation pathways are available and a mixture of ions is produced.available and a mixture of ions is produced.
mixture of ions of mixture of ions of different mass different mass gives separate peak gives separate peak for each m/zfor each m/z
intensity of peak intensity of peak proportional to proportional to percentage of each percentage of each ion of different ion of different mass in mixturemass in mixture
separation of peaks separation of peaks depends on relative depends on relative massmass
Principles of Electron-Impact Mass SpectrometryPrinciples of Electron-Impact Mass Spectrometry
++
++++
++
+
+
mixture of ions of mixture of ions of different mass different mass gives separate peak gives separate peak for each m/zfor each m/z
intensity of peak intensity of peak proportional to proportional to percentage of each percentage of each atom of different atom of different mass in mixturemass in mixture
separation of peaks separation of peaks depends on relative depends on relative massmass
++ ++ ++ ++
+ +
Principles of Electron-Impact Mass SpectrometryPrinciples of Electron-Impact Mass Spectrometry
2020 4040 6060 8080 100100 120 120
m/zm/z
m/z m/z = 78= 78
100100
8080
6060
4040
2020
00
Relative Relative intensityintensity
Some molecules undergo very little Some molecules undergo very little fragmentationfragmentation
Benzene is an example. The major peak Benzene is an example. The major peak corresponds to the molecular ion.corresponds to the molecular ion.
HH
HH HH
HHHH
HH
HH
HH HH
HHHH
HH
HH
HH HH
HHHH
HH
all H are all H are 11H and all H and all C are C are 1212CC
one C is one C is 1313CC one H is one H is 22HH
Isotopic ClustersIsotopic Clusters
7878 7979 7979
93.4%93.4% 6.5%6.5% 0.1%0.1%
2020 4040 6060 8080 100100 120 120
m/zm/z
100100
8080
6060
4040
2020
00
Relative Relative intensityintensity
112112
114114
Isotopic ClustersIsotopic Clustersin Chlorobenzenein Chlorobenzene
visible in peaks visible in peaks for molecular ionfor molecular ion
3535ClCl 3737ClCl
2020 4040 6060 8080 100100 120 120
m/zm/z
Relative Relative intensityintensity
7777
Isotopic ClustersIsotopic Clustersin Chlorobenzenein Chlorobenzene
no no mm//zz 77, 79 77, 79 pair; therefore ion pair; therefore ion responsible forresponsible formm//zz 77 peak does 77 peak does not contain Clnot contain Cl
HH
HH
HH
HH
HH ++
100100
8080
6060
4040
2020
00
Alkanes undergo extensive fragmentationAlkanes undergo extensive fragmentation
m/zm/z
DecaneDecane
142142
4343
5757
7171
8585
9999
CHCH33—CH—CH22—CH—CH22—CH—CH22—CH—CH22—CH—CH22—CH—CH22—CH—CH22—CH—CH22—CH—CH33
Relative Relative intensityintensity
100100
8080
6060
4040
2020
00
2020 4040 6060 8080 100100 120 120
Propylbenzene fragments mostlyPropylbenzene fragments mostlyat the benzylic positionat the benzylic position
2020 4040 6060 8080 100100 120 120
m/zm/z
Relative Relative intensityintensity
120120
9191 CHCH22—CH—CH22CHCH33
100100
8080
6060
4040
2020
00
13.2313.23
Molecular FormulaMolecular Formula
as aas a
Clue to StructureClue to Structure
Molecular WeightsMolecular Weights
One of the first pieces of information we try to One of the first pieces of information we try to obtain when determining a molecular obtain when determining a molecular structure is the molecular formula.structure is the molecular formula.
However, we can gain some information even However, we can gain some information even from the molecular weight. Mass from the molecular weight. Mass spectrometry makes it relatively easy to spectrometry makes it relatively easy to determine molecular weights.determine molecular weights.
The Nitrogen RuleThe Nitrogen Rule
A molecule with an A molecule with an odd number of odd number of nitrogens has an odd nitrogens has an odd molecular weight.molecular weight.
A molecule that A molecule that contains only C, H, contains only C, H, and O or which has and O or which has an even number of an even number of nitrogens has an nitrogens has an even molecular even molecular weight.weight.
NNHH22 9393
138138
NNHH22OO22NN
183183
NNHH22OO22NN
NNOO22
Exact Molecular WeightsExact Molecular Weights
CHCH33(CH(CH22))55CHCH33
HeptaneHeptane
CHCH33COCO
OO Cyclopropyl acetateCyclopropyl acetate
Molecular formulaMolecular formula
Molecular weightMolecular weight
CC77HH1616 CC55HH88OO22
100100 100100
Exact massExact mass 100.1253100.1253 100.0524100.0524
Mass spectrometry can measure exact Mass spectrometry can measure exact masses. Therefore, mass spectrometry can masses. Therefore, mass spectrometry can give molecular formulas.give molecular formulas.
Molecular FormulasMolecular Formulas
Knowing that the molecular formula of a Knowing that the molecular formula of a substance is Csubstance is C77HH1616 tells us immediately that is tells us immediately that is
an alkane because it corresponds to Can alkane because it corresponds to CnnHH22nn+2+2
CC77HH1414 lacks two hydrogens of an alkane, lacks two hydrogens of an alkane,
therefore contains either a ring or a double therefore contains either a ring or a double bond bond
Index of Hydrogen DeficiencyIndex of Hydrogen Deficiency
relates molecular formulas to multiple bonds relates molecular formulas to multiple bonds and ringsand rings
index of hydrogen deficiency = index of hydrogen deficiency =
1122
(molecular formula of alkane –(molecular formula of alkane – molecular formula of compound) molecular formula of compound)
Example 1Example 1
index of hydrogen deficiency index of hydrogen deficiency
C7H14C7H14
1122
(molecular formula of alkane –(molecular formula of alkane – molecular formula of compound) molecular formula of compound)
==
1122
(C(C77HH1616 – C – C77HH1414))==
1122
(2) = 1(2) = 1==
Therefore, one ring or one double bond.Therefore, one ring or one double bond.
Example 2Example 2
C7H12C7H12
1122
(C(C77HH1616 – C – C77HH1212))==
1122
(4) = 2(4) = 2==
Therefore, two rings, one triple bond,Therefore, two rings, one triple bond,two double bonds, or one double bond + one ring.two double bonds, or one double bond + one ring.
Oxygen has no effectOxygen has no effect
CHCH33(CH(CH22))55CHCH22OH (1-heptanol, COH (1-heptanol, C77HH1616O) has O) has
same number of H atoms as heptanesame number of H atoms as heptane
index of hydrogen deficiency = index of hydrogen deficiency =
1122
(C(C77HH1616 – C – C77HH1616O)O) = 0= 0
no rings or double bondsno rings or double bonds
Oxygen has no effectOxygen has no effect
index of hydrogen deficiency = index of hydrogen deficiency =
1122
(C(C55HH1212 – C – C55HH88OO22)) = 2= 2
one ring plus one double bondone ring plus one double bond
CHCH33COCO
OO Cyclopropyl acetateCyclopropyl acetate
If halogen is presentIf halogen is present
Treat a halogen as if it were hydrogen.Treat a halogen as if it were hydrogen.
CC CC
CHCH33
ClClHH
HH
CC33HH55ClCl
same index of hydrogensame index of hydrogendeficiency as for Cdeficiency as for C33HH66
Rings versus Multiple BondsRings versus Multiple Bonds
Index of hydrogen deficiency tells us the sum ofIndex of hydrogen deficiency tells us the sum ofrings plus multiple bonds.rings plus multiple bonds.
Catalytic hydrogenation tells us how many Catalytic hydrogenation tells us how many multiple bonds there are.multiple bonds there are.